Anyone who thinks they might be interested in fantasy novels <i>has</i> to read Piers Anthony. His early Xanth novels are brilliant.

Anyway, I don't think I can limit myself to one book. So I'll just go by series:

<i>The DragonLance Saga</i>, by Margaret Weis and Tracy Hickman
<i>Death Gate Cycle</i>, also by Weis/Hickman
<i>The Jack Frost Series</i>, by R.D. Wingfield
And of course, <i>The Hitchhiker's Guide to the Galaxy</i>, by Douglas Adams.

Honorable mentions:

Piers Anthony in general
David Eddings's <i>The Belgariad</i> and <i>The Mallorean</i>
Terry Goodkind's <i>Sword of Truth</i> series